cmd/git(feat[GitSubmoduleManager]): add GitSubmoduleManager and GitSubmoduleEntryCmd

why: Complete the Manager/Cmd pattern for submodule operations
what:
- Add GitSubmodule dataclass with name, path, url, sha, branch, status_prefix
- Add GitSubmoduleEntryCmd for per-submodule operations: init, update, deinit,
  set_branch, set_url, status, absorbgitdirs
- Add GitSubmoduleManager with collection operations: add, init, update, foreach,
  sync, summary, absorbgitdirs, _ls, ls, get, filter
- Add 'add' to GitSubmoduleCmdCommandLiteral
- Add self.submodules to Git class for easy access
- Fix return type assertions in get() methods for mypy

Author: tony